Hey Eduard,
I have been hesitant to write an answer because I'm far from an expert in either sketch up or blender. As you, I'm also currently in the process of finding sanity-preserving ways to work with sketch up models in blender. While I still have to tackle uv unwrapping, I think I am now proficient enough that I can address the other issues I had so far.
I think sketch up is an awesome tool for architectural design, which is why I thought it might me a good choice for modeling skylines assets. Turns out, I was wrong. My conclusion after spending way too much time on this: What happens in sketch up stays in sketch up. It's better that way :~)
However, since you have a lot of stuff already made in sketchup it's certainly feasible to make use of that.
Ignoring texture work, I've so far encountered four major problems with sketchup models:
1. Double sided faces
2. Unrealiable (collada-)export: Sometimes a lot of faces are just missing etc.
3. Normals are usually messed up
4. Aggressive triangulation during export
I got rid of the first two problems by using the obj-exporter plugin. The version I have is ancient and I don't have a download link anymore, but I think this is it:
http://sketchucation.com/pluginstore?pln=OBJexporter (requires registration). My version resides in the file menu and just dumps the obj/mtl file in the same directory the skp file is in. The exporter gets rid of double faces for you. I should also preserve your texture work but I have no experience with that. This reduces the number of annoying problems to two :~)
I ended up drawing a lot of additional edges in sketchup to partition all faces into mostly rectangular shapes:
The triangulation process will respect these lines and the result likes like this:
To undo the triangulation, I enter edit mode (TAB) while my model is selected (press a until nothing is selected, then right click the model). Then I go into the Mesh-Menu (usually in the lower left of the 3d-View), and select Clean Up/Limited Dissolve.
The last problem are the normals. Basically normals define the direction a face is facing. In the sketchup screenshot above the orange colored faces are facing outwards while the others are actually facing inwards. In sketchup this doesn't really matter because it uses double sided faces. In skylines (and virtually every other 3d game) however, only the faces that are facing the camera are drawn. To see how that looks like, you can enable "backface culling" in blender. Hit n to open the properties panel (make sure your mouse cursor is hovering over the 3d view). Click Shading/Backface Culling. If a parts of your building just disappeared, your normals are messed up. You can also directly enable displaying of normals. Make sure you are in edit mode and go to Mesh Display/Normals in the properties panel. Here, click on the icon that highlights a face and pick a size of 0.20. Like cyan hair from a pimple, you will now see the normals pointing in the direction the face is facing:
Now let's fix this. Disable backface culling for now. I find it useful however to turn it on every now and then to check my work. Now make sure that you are in edit mode and that your selection mode is set to faces (In the menu/tool-bar below the 3d view you have three icons with different selection modes: vertex, edge and face. Select the one shows a cube with a highlighted face.) Now start selecting faces with wrong normals (right click, shift selects multiple). When you have a few selected, press SPACE and begin to type "flip normals" in the search box until you see the option with the same name. Select that and the normals will be flipped. Now repeat the process. The next time you hit space you will see that the flip normals is already selected, so just hit enter to repeat the last operation. Depending on the model there are probably more efficient ways to do that (for instance, check out the Select Menu or try shift-g when a face is selected), but at least it works.
Eventually you should end up with a model that is suitable for uv-mapping. I'll write about that once I have some experience to share. Even without textures you can already export an fbx model and try it out in skylines' asset editor, by the way.
Regarding blender, there's no way around learning how to use it. There are great resources out there, e.g. Noob to Pro (wiki books). Youtube is also a good source, but don't think it won't take some time to get into it.
